[ServiceWorker] FetchEvent should returns integrity attribute

This patch is to pass the web-platform-test:fetch-request-resources.https.html.
Current failed cases(script_integrity_test and css_integrity_test) are caused
by integrity attribute data will not be returned in FetchEvent.

BUG=718935, 678905

Review-Url: https://codereview.chromium.org/2941883003
Cr-Commit-Position: refs/heads/master@{#487710}
44 files changed